The present invention generally relates to circuit boards and their fabrication. More particularly, this invention relates to a method for integrating stable resistive and capacitive materials with organic-based substrates, yielding resistors and capacitors that exhibit stability under temperature and humidity stress and over a wide range of resistance/capacitance values.
Inorganic-based thick-film resistors are formed of materials that can provide a wide range of stable resistor values, and therefore have been widely used to replace discrete resistors on ceramic substrates of hybrid electronic circuits. Such resistors are formed by printing, such as screen printing, a thick-film resistive paste or ink that is typically composed of a glass frit composition, an electrically-conductive material, various additives used to favorably affect the final electrical properties of the resistor, and an organic vehicle. After printing, the ink is sintered or fired at a high temperature, typically about 850 to 900xc2x0 C. The heating rate is controlled to first remove the organic vehicle, after which the glass frit composition is bonded together and to the ceramic substrate, forming a solid resistive mass that forms a thick-film resistor with a predictable and stable resistance when properly terminated. Other layers of material may then be printed over the resistor to yield a buried resistor in a multi-layer circuit board.
Thick-film inks of the type described above are fired at temperatures that organic substrates, such as those for printed wiring boards, cannot tolerate. Therefore, integrated resistors for organic substrates are typically formed from resistive materials such as NiP plated on copper which is subsequently laminated to the substrate, CrSi sputtered onto copper on polyimide, and polymer thick-film (PTF) inks that are printed on the substrate. PTF inks are generally composed of an electrically-conductive material (e.g., carbon) in an organic matrix material. After printing, PTF inks are heated to cure the matrix material to form an electrically-resistive film that adheres to the organic substrate. The temperature to which PTF materials must be heated is roughly about 200xc2x0 C., which can be tolerated by organic substrates for a few minutes.
Processes required for organic substrates and their resulting integrated resistors have certain disadvantages as compared to resistors and processes for ceramic substrates. The NiP and CrSi materials are expensive and can be practically produced only with low sheet resistivities (up to about 1000 ohms/square). Consequently, large-value resistors must be very long, introducing electromagnetic interference (EMI) problems and occupying relatively large areas on or within the organic substrate. PTF resistors can be cost-effective but are relatively unstable because the organic matrix continues to cure during subsequent exposures to high temperatures (e.g., above 100xc2x0 C.), causing drift in the resistance value. Extended cures at temperatures sufficient to eliminate this problem are not compatible with organic substrates.
In view of the above, it can be appreciated that integrated resistors that can be formed on organic substrates are often very large or less stable than integrated thick-film resistors formed on ceramic substrates. Similar differences can be seen between capacitors for ceramic and organic substrates. Stable integrated thick-film capacitors are formed on ceramic substrates using thick-film inks of high-dielectric constant (high xcex5r) materials (e.g., BaTiO3, perovskites) and an organic vehicle, the latter of which is burned off during firing. For organic substrates, high-xcex5r materials such as BaTiO3 are mixed with organic binders such as epoxy to form a composite that can be deposited and cured to form a capacitor. High-xcex5r materials and one of the capacitor electrodes may also be sputtered roll-to-roll onto copper on polyimide for organic substrates, or a high-xcex5r film (such as polyimide with BaTiO3) may have conductors sputtered onto both sides. Other deposition methods include vacuum deposition and solution coating. Lower xcex5r organic (polymer) materials are deposited in thin layers and cured to form thin capacitors in organic substrates. In some applications, the low total capacitance of capacitors formed from lower-xcex5r organic materials can replace enough discrete capacitors to be cost-effective. However, lower-xcex5r organic materials generally cannot form small capacitors of appreciable capacitance values. Consequently, capacitors formed from these materials have found limited application, such as power filters where entire planes are formed of these materials to decouple power everywhere on a substrate.
The higher-xcex5r materials mixed in organic matrix materials noted above achieve higher capacitance values, and can be quite effective for filtering power. However, capacitors used as filters on signals would be desirable, and for these applications only one of the electrodes (ground) can be in common, such that there is no benefit to be gained from sharing capacitance (as with power filtering). Considerable development would be required to make the higher-xcex5r materials in organic matrices a suitable capacitor material for integrating discrete capacitors in organic substrates. Finally, sputtered high-xcex5r materials, whether applied directly as an oxide or applied as a metal and anodized, currently suffer from defects in addition to having high material and processing costs.
From the above, it can be appreciated that resistors and capacitors have been integrated into ceramic substrates using inorganic-based materials and are able to have a wide range of stable resistance and capacitance values. However, the inorganic-based materials that provide these benefits have not been used with organic substrates because the high firing temperatures required are incompatible with organic substrates. Organic-based materials, whose processing temperatures (e.g., cure) are compatible with organic substrates, do not yield resistors and capacitors that are as stable as inorganic-based resistors and capacitors, and their relatively poor resistive and capacitive properties do not allow for stable resistance and high capacitance values. Accordingly, what is needed are stable resistors and capacitors that can be formed to have a wide range of resistance/capacitance values, yet can be integrated on organic substrates.
According to the present invention, there is provided a process for forming stable integrated resistors and capacitors on organic substrates. The resistors and capacitors are capable of a wide range of resistance and capacitance values, yet can be processed in a manner that does not detrimentally effect the organic substrate or entail complicated processing. Resistors and capacitors formed in accordance with this invention can be buried in a multi-layer circuit board.
The method of this invention generally entails the use of thick-film materials of the types used to form resistors and capacitors on ceramic substrates. The thick-film materials are applied to an electrically-conductive foil, such as a copper or stainless steel foil, and then heated to bond the thick-film material to the foil and form a solid mass. The solid mass will either be electrically resistive, capacitive or insulating, depending on the composition of the thick-film material. For example, the solid mass is electrically resistive if the thick-film material contains an inorganic composition, an electrically-conductive material, and an organic vehicle, but would be capacitive if the thick-film material contains a high-xcex5r constituent. To be useful as a capacitor, the dielectric constant of the solid mass should be very high, such as with the BaTiO3 family of materials. For a capacitor, a second conductor layer is applied to the solid mass such that the solid high-xcex5r mass is sandwiched between with the conductive foil and the second conductor layer.
According to the invention, the conductive foil (with the solid mass and, if present, the second conductor layer) is then laminated to an organic substrate, such that the solid mass is attached to the organic substrate and preferably embedded in the organic substrate. The foil is etched to form at least one terminal that contacts the solid mass and thereby completes the passive electrical component. Finally, the resulting composite or core may be laminated with other layers of materials of the type used to form organic substrates. The sequence of layup of these materials may result in the passive electrical component being buried or remaining on the substrate surface.
In the above example, the thick-film material can essentially be the same as those used to form integrated inorganic-based resistors and capacitors on ceramic substrates, in which case the heating step entails firing the material to first burn off an organic vehicle and then bond the remaining material together and to the foil. Accordingly, the resulting resistor or capacitor is characterized by a stable resistance or capacitance value that is within a wide range of possible values. Alternatively, thick-film resistors and capacitors can be formed with PTF materials used in the prior art to form integrated resistors and capacitors on organic substrates. However, the instability of these resistors and capacitors noted with the prior art is overcome with this invention by performing the heating step as an extended cure at a temperature sufficient to completely cure the organic matrix prior to laminating the foil to the organic substrate. This feature of the invention also permits the use of other organic materials, such as polyimides, to be used as the matrix material for an organic-based resistor or capacitor of this invention.
In view of the above, it can be seen that high-temperature processing that would be incompatible with an organic substrate is only performed with resistive and capacitive materials, a foil such as copper that is relatively resistant to elevated processing temperatures, and any interface materials that might be included. Only then are the high-temperature materials laminated to an organic substrate, with subsequent processing of the organic substrate being limited to temperatures that the substrate will safely tolerate. Accordingly, the present invention provides the benefit of passive electrical components with stable, wide-ranged electrical properties as integrated components of an organic substrate. Advantages attributable to this capability include higher circuit densities, improved EMI performance, reduced circuit board size requirements, and lower production costs.
Other objects and advantages of this invention will be better appreciated from the following detailed description.